Use BLOCK_REGION_NONE instead of -1

Problem/Motivation

The attached patch is probably self-explanatory.

block_admin_configure() uses the value -1 to test whether a block is in a region or not.
Maybe we should use the constant BLOCK_REGION_NONE instead of -1, in case one day we want to change the value of BLOCK_REGION_NONE.

Proposed resolution

Use BLOCK_REGION_NONE instead of -1. See patch.

Remaining tasks

Review/test/advise about the patch.